The present invention relates to printed media production and in particular ink jet printers.
Ink jet printers are a well-known and widely used form of printed media production. Ink is fed to an array of digitally controlled nozzles on a printhead. As the print head passes over the media, ink is ejected from the array of nozzles to produce an image on the media.
Printer performance depends on factors such as operating cost, print quality, operating speed and ease of use. The mass, frequency and velocity of individual ink drops ejected from the nozzles will affect these performance parameters.
Recently, the array of nozzles has been formed using microelectromechanical systems (MEMS) technology, which have mechanical structures with sub-micron thicknesses. This allows the production of printheads that can rapidly eject ink droplets sized in the picolitre (xc3x9710xe2x88x9212 liter) range.
While the microscopic structures of these printheads can provide high speeds and good print quality at relatively low costs, their size makes the nozzles extremely fragile and vulnerable to damage from the slightest contact with fingers, dust or the media substrate. This can make the printheads impractical for many applications where a certain level of robustness is necessary. Furthermore, a damaged nozzle may fail to eject the ink being fed to it. As ink builds up and beads on the exterior of the nozzle, the ejection of ink from surrounding nozzle may be affected and/or the damaged nozzle will simply leak ink onto the printed substrate. Both situations are detrimental to print quality.
To address this, an apertured guard may be fitted over the nozzles to shield them against damaging contact. Ink ejected from the nozzles passes through the apertures on to the paper or other substrate to be printed. However, to effectively protect the nozzles the apertures need to be as small as possible to maximize the restriction against the ingress of foreign matter while still allowing the passage of the ink droplets. Preferably, each nozzle would eject ink through its own individual aperture in the guard. However, given the microscopic scale of MEMS devices, slight misalignments between the guard and the nozzles will obstruct the path of the ink droplets.
According to a first aspect, the present invention provides a printhead for an ink jet printer, the printhead including:
an array of nozzles for ejecting ink onto media to be printed; and
alignment formations configured for engagement with complementary formations on an apertured nozzle guard having an array of ink apertures corresponding to the array of nozzles; wherein;
engagement between the alignment formations and the complementary formations holds the apertures in registration with the nozzles such that the guard does not obstruct the normal trajectory of ink ejected from the nozzles onto the media.
In this specification the term xe2x80x9cnozzlexe2x80x9d is to be understood as an element defining an opening and not the opening itself.
According to another aspect, the present invention provides a printhead assembly for an inkjet printer, the printhead assembly including:
a printhead having an array of nozzles for ejecting ink onto media to be printed; and,
an apertured nozzle guard having an array of ink apertures corresponding to the array of nozzles;
the printhead further including alignment formations inter-engaged with complementary formations on the apertured nozzle guard to hold the apertures in registration with the nozzles such that the guard does not obstruct the normal trajectory of ink ejected from the nozzles onto the media.
Preferably, each of the nozzles in the array is individually aligned with one of the ink apertures in the nozzles guard. However, some forms of the invention may have two or more of the nozzles sharing one of the ink passages of the nozzle guard.
In some embodiments of the invention, the array of nozzles is formed on a silicon substrate incorporating the alignment formations. The nozzle guard may have a shield containing the array of ink apertures, the shield being spaced from the silicon substrate by integrally formed struts extending from the shield for engagement with the alignment formations. In one convenient form, the alignment formations are spaced ridges on the silicon substrate positioned to slidingly engage the sides of the struts to maintain the apertures in alignment with the nozzle array.
In another form, the alignment formations are recesses in the substrate positioned to slidingly engage the sides of the struts to maintain the nozzle guard in alignment with the nozzle array. Of course other forms of the invention may have struts integrally formed and extending from the silicon substrate to engage continuous ridges or recesses formed in the nozzles guard.
In a particularly preferred embodiment, the alignment formations are formed during the production of the array of nozzles. It is envisaged that this system of production will align the nozzles and the passages to within 0.1 micron. Furthermore, it is preferable to form the nozzle guard from silicon for ease and accuracy of micro-machining, strength, rigidity and a coefficient of thermal expansion that matches that of the printhead.
The alignment formations necessarily use up a proportion of the surface area of the printhead, and this adversely affects the nozzle packing density. The extra printhead chip area required adds to the cost of manufacturing the chip. However, in situations where conventional methods of assembling the printhead and the nozzle guard is likely to provide the required accuracy, the present invention will effectively account for a relatively high nozzle defect rate.
The nozzle guard may further include fluid inlet openings for directing fluid through the passages, to inhibit the build up of foreign particles on the nozzle array. In this embodiment, the fluid inlet openings may be arranged in the struts.
It will be appreciated that, when air is directed through the openings, over the nozzle array and out through the passages, the build up of foreign particles on the nozzle array is inhibited.
The fluid inlet openings may be arranged in the support element remote from a bond pad of the nozzle array.
By providing a nozzle guard for the printhead, the nozzle structures can be protected from being touched or bumped against most other surfaces. To optimize the protection provided, the guard forms a flat shield covering the exterior side of the nozzles wherein the shield has an array of passages big enough to allow the ejection of ink droplets but small enough to prevent inadvertent contact or the ingress of most dust particles. By forming the shield from silicon, its coefficient of thermal expansion substantially matches that of the nozzle array. This will help to prevent the array of passages in the shield from falling out of register with the nozzle array. Using silicon also allows the shield to be accurately micro-machined using MEMS techniques. Furthermore, silicon is very strong and substantially non-deformable.